The present invention relates to a strain detector, a method for manufacturing the same and a plating bath therefor.
FIG. 1 shows a conventional strain detector disclosed, for example, in Japanese Patent Disclosure Sho No. 57-211030. In FIG. 1, reference numeral 1 denotes a shaft-shaped driven member for receiving a torque, numeral 2 denotes a pair of magnetostrictive layers made of a high permeability soft magnetic material fixed in a band shape to the driven member 1 to vary its permeability in response to the quantity of an internal strain generated by a torque applied to the driven member 1, and numeral 3 denotes a pair of detecting coils provided on the outer peripheries of the magnetostrictive layers 2 for detecting the quantity of the variation in its permeability. Each magnetostrictive layer 2 is composed of a plurality of rectangular magnetic pieces so arranged as to be laterally symmetrical at .+-.45.degree. in its extending direction and hence to form an angle of 90.degree. therebetween.
The operation of the strain detector constructed as described above will be described. When a torque is externally applied to the driven member 1, a main stress is generated in the long axial direction of the magnetostrictive layers 2 made of the magnetic piece groups as a main axis. If the main stress is, for example, a tensile force of the magnetic piece group of one magnetostrictive layer 2, it is a compressive force of the piece group of the other magnetostrictive layer 2. Generally, when a stress is applied to a magnetic material in which its magnetostrictive constant is not zero, its magnetic property is varied, with the result that its permeability is altered. This phenomenon is used for a so-called magnetostrictive converter for converting a mechanical energy into an electrical energy. When a magnetic element is deformed, it corresponds to Villari effect in which its permeability is varied in response to the quantity of its deformation. It is known that, when the magnetostrictive constant of the quantity which quantitatively represents the magnitude of the magnetostriction is positive, its permeability increases if a tensile force is acted, while it decreases if a compressive force is acted, whereas when the magnetostrictive constant is negative, vice versa. Accordingly, the deformation responsive to the quantity of the torque applied externally is detected as the variation in the permeability of the magnetostrictive layer 2, and the variation in the permeability is detected as the change of the magnetic impedance by the detecting coil 3 thereby to detect the quantity of the torque applied to the driven member 1 and the quantity of the strain upon the application of the torque.
In a conventional strain detector disclosed in Japanese Patent Disclosure Sho No. 59-164931, a masking pattern is wound on the outer periphery of a driven member applied with a stress, and Ni is plated thereon by electrolytic plating to form magnetostrictive layers made of magnetic piece groups.
However, in such a conventional strain detector, the magnetic layers are formed by Ni plating by electrolytic plating, the electrolytic plating generally has crystalline characteristic to form large magnetic domains, large coercive force Hc, and relatively low magnetostrictive sensitivity. As shown in FIG. 2, its output characteristic generates a hysteresis. Since its permeability is not so large, the Ni plating also decreases its detecting sensitivity. Further, the skin depth of its magnetic flux, i.e., the penetrating depth .delta. of the magnetic flux is ##EQU1## (.omega.: the angular frequency of a current applied to the detecting coil, .sigma. :conductivity, .mu.: permeability). Since the .mu., .sigma. are not so large in the case of the Ni plating, the skin depth .delta. becomes relatively large, the magnetic flux does not concentrically pass in the magnetic layers 5 and 6, and its detecting sensitivity decreases.
The conventional method of forming the magnetostrictive layers 2 includes the step of forming a so-called chevron pattern by Ni electrolytic plating using a mask pattern or etching after Ni electrolytic plating, but it is not easy to accurately form the magnetic pieces by such pattern plating.
Further, the magnetostrictive layers 2 of uniform thickness are scarcely obtained by the concentration of an electric field by electrolytic plating, and a detecting error feasibly occurs due to the difference of thermal stresses. Moreover, a large-scale apparatus is required by electrolytic plating and it lacks its mass productivity.